This event log was recorded during R/V Endeavor cruise EN734 in the central North Atlantic Ocean, during July and August 2025. The cruise track included six process stations, four of which comprise an offshore transect (PS1, PS2, PS3, PS4) and two of which were inside the lower reaches of Chesapeake Bay (PS5, PS6). Experimental manipulations to measure nitrogen (N) cycle processes were carried out at these stations. The sampling strategy was tailored to the process under investigation, e.g., photosynthesis and photochemical processes in surface waters, and depth profiles for most standard rates. Complete depth profiles were measured for nitrification, while CO2 and nitrogen nutrient (nitrate, ammonium, urea) uptake rates focused on the euphotic zone.  One additional short station (PS6) was sampled only for nitrification and nitrate uptake. Nitrous oxide consumption and photochemical production of nitrous oxide were measured in surface waters at PS1-PS5. Zooplankton net tows were carried out at PS1 – PS5. This dataset includes an event log documenting over-the-side sampling activities (date, time, station, position, and observer notes) combined with CTD and Niskin bottle data (depth, temperature, salinity, sigma-theta, dissolved oxygen, beam attenuation, transmission, chlorophyll fluorescence, and PAR) collected at each process station during the cruise.

Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Curation Notes
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Event Log

The cruise track included six process stations, four of which comprised a transect along 36 N from the mid Sargasso Sea inshore towards Chesapeake Bay, including one in the Gulf Stream (PS1 - PS4). The other two stations (PS5, PS6) were inside Chesapeake Bay. Experimental manipulations to measure nitrogen (N) cycle processes were carried out at these stations. The sampling strategy was tailored to the process under investigation, e.g., photosynthesis and photochemical processes in surface waters, and depth profiles for most standard rates. Complete depth profiles were measured for nitrification, while CO₂ and nitrogen nutrient (nitrate, ammonium, urea) uptake rates focused on the euphotic zone. One additional short station (PS6) was sampled only for nitrification and nitrate uptake. Nitrous oxide consumption and photochemical production of nitrous oxide were measured in surface waters at PS1-PS5. 

CTD/Niskin bottle

A Seabird SBE-911+ was used with 12 30-L Niskin bottles. 

Sensors: 

1.     Temperature Sensor, SensorID="55"; serial number: 4695
2.     Conductivity Sensor, SensorID="3"; serial number: 0618
3.     Pressure, Digiquartz with TC, SensorID="45"; serial number: 0444
4.     Temperature Sensor, SensorID="55"; serial number: 4130
5.     Conductivity Sensor, SensorID="3"; serial number: 2822
6.     Transmissometer, WET Labs C-Star, SensorID="71"; serial number: 969DR
7.     Fluorometer, WET Labs ECO-AFL/FL, SensorID="20"; serial number: 4028
8.     Altimeter Sensor, SensorID="0"; serial number: 49898
9.     PAR/Irradiance, Biospherical/Licor, SensorID="42"; serial number: 70709
10.     Oxygen, SBE 43, SensorID="38"; serial number: 1230
11.     Oxygen, SBE 43, 2; SensorID="38"; serial number: 1648
12.     User Polynomial Sensor, SensorID="61"; serial number: 783
13.     SPAR, Biospherical/Licor, SensorID="51"; serial number: 20121

Software Version Seasave V 7.27.0.46

The data have been preliminarily processed by the ship’s technician using Sea-Bird recommended default parameters. Raw CTD data were processed using SBEDataProcessing-Win32 software. These files should not be taken as the final processed product for analysis. Reference the Seasoft Data Processing manual for more information. We did not perform additional processing of the data in the bottle files.

This section documents curation actions performed prior to publication review with the submitter, and additional information relevant to understanding and reusing this dataset. It distinguishes changes made to the submitted (meta)data from unresolved issues and/or enhancements that improve future reuse and interoperability.

CURATION ACTIONS PERFORMED ON DATA

- Loaded EN734_2025_EventLog.xlsx (sheet 1, header row 1) with strings cast strategy, empty string and "nd" set as missing values
- Loaded EN734_BottleFilesCombined.xlsx (sheet 1, header row 1), preserving displayed cell formatting, with "", "nd", "N/A" set as missing values, named en734_bottle_files_combined
- Created Event column in en734_bottle_files_combined by formatting Cast number as "CTD" plus zero-padded 2-digit cast number
- Converted Month/Day/Year/Time columns into CTD_Collection_Datetime_UTC (UTC, ISO 8601 format)
- In event log table, combined Lat Deg (N) and Lat Min (N) into temporary lat_dms_temp string, skipping rows where lat was "N/A"
- Combined Long Deg (W) and Long Mins (W) into temporary lon_dms_temp string, skipping rows where lon was "N/A"
- Converted lat_dms_temp from degrees-decimal_minutes to decimal degrees as Latitude_Event_Log, forcing N directional, skipping missing rows
- Converted lon_dms_temp from degrees-decimal_minutes to decimal degrees as Longitude_Event_Log, forcing W directional (negative), skipping missing rows
- Fixed two Date typos in event log: "7/224/2025" to "7/24/2025" and "7/27/2-25" to "7/27/2025"
- Converted Date + Local Time (in water) into Deploy_Datetime_Local (datetime, "%Y-%m-%dT%H:%M")
- Converted Date + UTC time column into Deploy_Datetime_UTC (datetime, UTC, "%Y-%m-%dT%H:%MZ")
- Deleted temporary/intermediate columns: Date, Local Time (in water), UTC, Lat Deg (N), Lat Min (N), Long Deg (W), Long Mins (W), lat_dms_temp, lon_dms_temp from event log table
- Rounded Latitude_Event_Log and Longitude_Event_Log to 5 decimal places, no trailing zeros
- Deleted original Date, Month, Day, Year, Time columns from en734_bottle_files_combined
- Renamed Latitude to Latitude_CTD_Collection and Longitude to Longitude_CTD_Collection in bottle files table
- Joined event log table (source, keyed on Event, deleted after join) into en734_bottle_files_combined (target, keyed on Event) as full-outer join, pulling in Air Temp (°C), Deploy_Datetime_Local, Deploy_Datetime_UTC, Event, Latitude_Event_Log, Longitude_Event_Log, Notes, Observer, Rosette, Station, Surface Temp (°C) using first-value aggregation
- Renamed fields with spaces/special characters for BCO-DMO naming compliance: "Potential T" to Potential_T, "Beam Attenuation" to Beam_Attenuation, "Surface Temp (°C)" to Surface_Temp, "Air Temp (°C)" to Air_Temp
- Reordered columns into final logical sequence (Event, CTD_Collection_Datetime_UTC, coordinates, deploy times/coordinates, cast/bottle/CTD parameters, station/environmental/metadata columns)
- Renamed Latitude_CTD_Collection to CTD_Collection_Latitude, Longitude_CTD_Collection to CTD_Collection_Longitude, Latitude_Event_Log to Deploy_Latitude, Longitude_Event_Log to Deploy_Longitude
- Replaced µ (micro sign) character with "u" in Notes column
- Output file as 1005482_v1_en734_event_log_ctd.csv

CURATION ACTIONS PERFORMED ON METADATA

- BCO-DMO's standard metadata entry and text formatting steps were performed. See: https://www.bco-dmo.org/how-to/standard-curation-edits

ISSUES POTENTIALLY IMPACTING REUSE

- An unexpected date is used for CTD05.

Coverage: North Atlantic Ocean

NSF Award Abstract:
Nitrous oxide (N2O) is a trace component of Earth’s atmosphere. It is a strong greenhouse gas, whose concentration has been increasing since the industrial revolution, and contributes about 6% to the total greenhouse effect. N2O is also implicated in ozone depletion in the stratosphere. The ocean is a net source of N2O to the atmosphere. N2O is produced and consumed in the ocean by microbes as part of the nitrogen cycle. The conversion of N2O to N2 (dinitrogen gas) is the only known biological sink for N2O and is catalyzed by an enzyme that is strongly inhibited by oxygen, so N2O consumption has been thought to be confined to oxygen-free environments. Thus, it was surprising to find that the genetic capability for N2O consumption was both present and active in the surface ocean, where oxygen is abundant. Experiments showed that surface ocean microbes rapidly consume N2O when oxygen is removed. It appears that the microbes can use N2O as an alternative to oxygen for respiration. Why would it be advantageous to retain this capacity in fully oxygenated surface water? Does this potential N2O consumption constitute an actual sink for N2O, which might reduce the net transfer of N2O from the ocean to the atmosphere?

The reduction of nitrous oxide (N2O) to N2 is considered to be an obligately anaerobic process, usually restricted to anoxic environments in water and sediments. Previous work showed that the genes (nosZ) encoding the N2O reductase enzyme were both present and expressed in the surface ocean, and that surface ocean samples rapidly reduce N2O to N2 when oxygen is removed. The nosZ genes in surface waters appear to belong almost exclusively to microbes that do not perform the upstream steps in denitrification – they are facultative N2O respirers. The research proposed here will investigate the factors that might control or stimulate N2O reduction in surface waters and characterize the microbes responsible for the process. Experiments will test the hypothesis that N2O respirers are versatile heterotrophs whose activity is related to organic matter supply and possibly associated with particulate material or linked to in situ primary production. Researchers will perform 15N tracer incubations to measure the rate of N2O reduction and its response to various kinds of organic substrates, including in situ particulate material and fresh phytoplankton exudates. The quantity and community composition of nosZ-containing microbes will be determined using a suite of molecular biological methods – necessary because the nosZ gene is so diverse that previous methods have likely underestimated both its abundance and diversity and may not have identified the main microbes responsible for the process. Draft genomes of nosZ-containing microbes will be characterized to investigate their carbon metabolism and link their lifestyles to organic matter supply or phytoplankton. One field expedition is planned to investigate the significance of N2O reduction in the North Atlantic subtropical gyre. If N2O consumption occurs even at low rates over vast regions of the surface ocean, its impact on the overall N2O budget of the ocean and atmosphere could be large.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.